作业帮matlab 求-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2的极值syms x yf='-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2';fx=diff(f,x)fy=diff(f,y)x0=roots([30,90,-120,-360])y0=roots([0])fxx=diff(f,x,2);fyy=diff(f,y,2);fxy=diff(f,x,y);A=(fxx)*(fyy);x=2;y=0;a1=eval(A)b1
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matlab 求-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2的极值syms x yf='-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2';fx=diff(f,x)fy=diff(f,y)x0=roots([30,90,-120,-360])y0=roots([0])fxx=diff(f,x,2);fyy=diff(f,y,2);fxy=diff(f,x,y);A=(fxx)*(fyy);x=2;y=0;a1=eval(A)b1
matlab 求-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2的极值
syms x y
f='-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2';
fx=diff(f,x)
fy=diff(f,y)
x0=roots([30,90,-120,-360])
y0=roots([0])
fxx=diff(f,x,2);
fyy=diff(f,y,2);
fxy=diff(f,x,y);
A=(fxx)*(fyy);
x=2;
y=0;
a1=eval(A)
b1=eval(fxx)
c1=eval(f)
x=-3;
y=0;
a2=eval(A)
b2=eval(fxx)
c2=eval(f)
x=-2;
y=0;
a3=eval(A)
b3=eval(fxx)
c3=eval(f)
总是出现：
Error using ==> eval
Undefined function or variable 'xy'.
是哪里错了呢
第十行应该是
A=(fxx)*(fyy)-(fxy)^2;
少打了
matlab 求-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2的极值syms x yf='-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2';fx=diff(f,x)fy=diff(f,y)x0=roots([30,90,-120,-360])y0=roots([0])fxx=diff(f,x,2);fyy=diff(f,y,2);fxy=diff(f,x,y);A=(fxx)*(fyy);x=2;y=0;a1=eval(A)b1
第一行f='-120*x^3-30*x^4+18*x^5+5*x^6-30*xy^2';
改为：f='-120*x^3-30*x^4+18*x^5+5*x^6-30*x*y^2';
最后一项x,y之间少了个*号